Transformers enjoy widespread use as energy coupling devices. Most transformers have a primary and a secondary winding. Some transformers have multiple secondary windings, and some have multiple taps on a single secondary winding. A particular interest here are transformers having a single primary winding, and a single secondary winding.
Transformers typically provide isolation between the input signal applied to the primary winding, and the output signal which results on the secondary winding. Varying degrees of isolation are provided by each transformer based on the frequency response of the primary and secondary windings. Transformers also enable a user to easily increase or decrease the voltage level by varying the ratio of the number of turns contained in the primary winding with respect to the number of turns contained in the secondary winding. Classic examples of this include the use of a transformer in a television set to generate high voltages which are applied between the electron gun and the phosphor screen of the cathode ray tube. These are typically referred to as flyback transformers, and have a large difference in the turns ratio of the primary and secondary coils. The primary winding typically contains one or two turns of a coil which pass through the coil of a secondary winding having more than 10 times as many turns. This is necessary in order to increase the voltage across the secondary of the transformer to 10,000 to 50,000 volts from an input winding which has approximately 330 volts (peak to peak) applied across it (this corresponds to 110-120 volts RMS which is typically provided in a household appliance outlet.
Other applications of transformers include step down transformers which supply power to low power devices such as battery chargers, toys, and computer peripherals. For a time these were referred to as filament transformers because they provided a voltage suitable to drive the filament of a vacuum tube. Step down transformers of this type typically reduce the incoming AC signal of approximately 330 volts AC (peak to peak) to 8 volts (peak to peak), or 12 volts (peak to peak). A rectifier or other appropriate low voltage AC to DC converter then provides the required DC wave form to operate the particular toy or appliance.
Additionally, transformers employ a wide range of coupling techniques. Flyback transformers are typically formed by first wrapping a tightly wound secondary coil around a central bobbin which is between 1/4 and 11/2 inches in diameter. The wire that forms the secondary transformer is referred to as Formvar wire and is typically a copper wire which is coated with varnish, shellack, or another insulating substance. Aluminum wire is used in some applications. The bobbin is then removed, and the entire secondary winding is covered with a protective coating such as a high temperature wax, or various cloth or mylar tapes which are then coated with varnish or shellack. This maintains the form of the secondary winding. A primary winding is formed by passing a piece of wire once or twice through the center of the secondary winding. This primary winding is wrapped loosely around the secondary winding. The diameter of the wire of the secondary winding varies from approximately 22 gage to 12 gage depending on the particular application. The primary winding wire is typically coated with plastic or other suitable, relatively heavy duty insulator.
Coupling material may be added to enhance the efficiency of operation of the transformer. In the flyback transformer, no coupling material is usually inserted in between, or surrounding either the primary or secondary coils.
In other types of transformers, including certain filament transformers, the primary and secondary windings are each formed in the same manner as the secondary winding for the flyback transformer. Both of these windings are then placed around a ferrite core which provides magnetic coupling between the two windings. In some configurations, the windings are further encased in ferrite or iron material in order to enhance the coupling of the primary and secondary windings to provide greater efficiency in the transfer of energy between the primary and secondary windings.
Designing transformers also involves accepting a trade off in certain operating parameters. Electromagnetic coupling is typically more efficient at higher frequencies. Unfortunately, the inductive characteristics of the wire used tend to decrease the efficiency of the wire as the frequency increases. Additionally, ferrite material which is typically used as a core provides a greater efficiency for magnetic coupling at lower frequencies, and a poorer efficiency at higher frequencies due to the increased rate of change of the magnetic field. Thus, most transformer designs make strategic trade offs between the operating frequency and materials used to construct the transformer.
Design of high frequency transformers is difficult, and is usually hampered by the parasitic characteristics presented by the conductive material used to form the primary and secondary windings. The coupling material also presents difficulties in use.